1. CPS 506Comparative Programming Languages Control Structures

2. Topics • Introduction • Selection Statements • Iterative Statements • Unconditional Branching • Guarded Commands • Conclusions

3. Control Structure • A control structure is a set of control statements to • Indicate the order of instruction evaluations • Indicate which statements should be evaluated • Indicate the flow a the program

4. Control Structure • Control Structure types • Continuation at a different statement (unconditional branch or jump) • Executing a set of statements only if some condition is met (choice, i.e. conditional branch) • Executing a set of statements zero or more times, until some condition is met (i.e. loop - the same as conditional branch) • Executing a set of distant statements, after which the flow of control usually returns (subroutines) • Stopping the program, preventing any further execution (unconditional halt)

5. Selection Statements • A selection statement provides the means of choosing between two or more paths of execution • Two general categories: • Two-way selectors • Multiple-way selectors

6. Two-Way Selection Statements • General form: if control_expression then clause else clause • Design Issues: • What is the form and type of the control expression? • How are the then and else clauses specified? • How should the meaning of nested selectors be specified?

7. The Control Expression • If the thenreserved word or some other syntactic marker is not used to introduce the then clause, the control expression is placed in parentheses • In C89, C99, Python, and C++, the control expression can be arithmetic • In languages such as Ada, Java, Ruby, and C#, the control expression must be Boolean

8. Clause Form • In many contemporary languages, the then and else clauses can be single statements or compound statements • In Perl, all clauses must be delimited by braces (they must be compound) • Python uses indentation to define clauses if x > y : x = y print "case 1"

9. Nesting Selectors • Java example if (sum == 0) if (count == 0) result = 0; else result = 1; • Which if gets the else? • Java's static semantics rule: else matches with the nearest if

10. Nesting Selectors (continued) • To force an alternative semantics, compound statements may be used: if (sum == 0) { if (count == 0) result = 0; } else result = 1; The above solution is used in C, C++, and C# • Perl requires that all then and else clauses to be compound

11. Nesting Selectors (continued) • Statement sequences as clauses: Ruby if sum == 0 then if count == 0 then result = 0 else result = 1 end end

12. Nesting Selectors (continued) • Python if sum == 0 : if count == 0 : result = 0 else : result = 1

13. Multiple-Way Selection Statements • Allow the selection of one of any number of statements or statement groups • Design Issues: • What is the form and type of the control expression? • How are the selectable segments specified? • Is execution flow through the structure restricted to include just a single selectable segment? • How are case values specified? • What is done about unrepresented expression values?

14. Multiple-Way Selection: Examples • C, C++, and Java switch (expression) { case const_expr_1: stmt_1; … case const_expr_n: stmt_n; [default: stmt_n+1] }

15. Multiple-Way Selection: Examples • Design choices for C’s switch statement • Control expression can be only an integer type • Selectable segments can be statement sequences, blocks, or compound statements • Any number of segments can be executed in one execution of the construct (there is no implicit branch at the end of selectable segments) • default clause is for unrepresented values (if there is no default, the whole statement does nothing)

16. Multiple-Way Selection: Examples • C# • Differs from C in that it has a static semantics rule that disallows the implicit execution of more than one segment • Each selectable segment must end with an unconditional branch (goto or break) • Also, in C# the control expression and the case constants can be strings

17. Multiple-Way Selection: Examples • Ada case expression is when choice list => stmt_sequence; … when choice list => stmt_sequence; when others => stmt_sequence;] end case; • More reliable than C’s switch (once a stmt_sequence execution is completed, control is passed to the first statement after the case statement

18. Multiple-Way Selection: Examples type Directions is (North, South, East, West); Heading : Directions; case Heading is when North => Y := Y + 1; when South => Y := Y - 1; when East => X := X + 1; when West => X := X - 1; end case;

19. Multiple-Way Selection: Examples • Ada design choices: • Expression can be any ordinal type • Segments can be single or compound • Only one segment can be executed per execution of the construct • Unrepresented values are not allowed • Constant List Forms: • A list of constants • Can include: • Subranges • Boolean OR operators (|)

20. Multiple-Way Selection: Examples • Ruby has two forms of case statements • One form uses when conditions leap = case when year % 400 == 0 then true when year % 100 == 0 then false else year % 4 == 0 end • The other uses a case value and when values case in_val when -1 then neg_count++ when 0 then zero_count++ when 1 then pos_count++ else puts "Error – in_val is out of range" end

21. Multiple-Way Selection Using if • Multiple Selectors can appear as direct extensions to two-way selectors, using else-if clauses, for example in Python: if count < 10 : bag1 = True elif count < 100 : bag2 = True elif count < 1000 : bag3 = True

22. Multiple-Way Selection Using if • The Python example can be written as a Ruby case case when count < 10 then bag1 = true when count < 100 then bag2 = true when count < 1000 then bag3 = true end

23. Iterative Statements • The repeated execution of a statement or compound statement is accomplished either by iteration or recursion • General design issues for iteration control statements: • How is iteration controlled? • Where is the control mechanism in the loop?

24. Counter-Controlled Loops • A counting iterative statement has a loop variable, and a means of specifying the initial and terminal, and stepsize values • Design Issues: • What are the type and scope of the loop variable? • Should it be legal for the loop variable or loop parameters to be changed in the loop body, and if so, does the change affect loop control? • Should the loop parameters be evaluated only once, or once for every iteration?

25. Iterative Statements: Examples • FORTRAN 95 syntax DO label var = start, finish [, stepsize] • Stepsizecan be any value but zero • Parameters can be expressions • Design choices: • Loop variable must be INTEGER • The loop variable cannot be changed in the loop, but the parameters can; because they are evaluated only once, it does not affect loop control • Loop parameters are evaluated only once

26. Iterative Statements: Examples • FORTRAN 95 : a second form: [name:] Do variable = initial, terminal [,stepsize] … End Do [name] • Cannot branch into either of Fortran’s Do statements

27. Iterative Statements: Examples • Ada for var in [reverse] discrete_range loop ... end loop for I in Integer range 1 .. 10 loop ... end loop

28. Iterative Statements: Examples • Ada (con’t) • Design choices: • Type of the loop variable is that of the discrete range (A discrete range is a sub-range of an integer or enumeration type). • Loop variable does not exist outside the loop • The loop variable cannot be changed in the loop, but the discrete range can; it does not affect loop control • The discrete range is evaluated just once • Cannot branch into the loop body

29. Iterative Statements: Examples • C-based languages for ([expr_1] ; [expr_2] ; [expr_3]) statement • The expressions can be whole statements, or even statement sequences, with the statements separated by commas • The value of a multiple-statement expression is the value of the last statement in the expression • If the second expression is absent, it is an infinite loop

30. Iterative Statements: Examples • C-based languages (con’t) • Design choices: • There is no explicit loop variable • Everything can be changed in the loop • The first expression is evaluated once, but the other two are evaluated with each iteration

31. Iterative Statements: Examples • C++ differs from C in two ways: • The control expression can also be Boolean • The initial expression can include variable definitions (scope is from the definition to the end of the loop body) • Java and C# • Differs from C++ in that the control expression must be Boolean

32. Iterative Statements: Examples • Python for loop_variable in object: - loop body [else: - else clause] for i in range(1, 5): print i else: print 'The for loop is over'

33. Iterative Statements: Examples • Python (con’t) • The object is often a range, which is either a list of values in brackets ([2, 4, 6]), or a call to the range function (range(5), which returns 0, 1, 2, 3, 4) • The loop variable takes on the values specified in the given range, one for each iteration • The else clause, which is optional, is executed if the loop terminates normally

34. Iterative Statements: Logically-Controlled Loops • Repetition control is based on a Boolean expression • Design issues: • Pretest or posttest? • Should the logically controlled loop be a special case of the counting loop statement or a separate statement?

35. Iterative Statements: Logically-Controlled Loops: Examples • C and C++ have both pretest and posttest forms, in which the control expression can be arithmetic: while (ctrl_expr) do loop body loop body while (ctrl_expr) • Java is like C and C++, except • The control expression must be Boolean • The body can only be entered at the beginning -- Java has no goto

36. Iterative Statements: Logically-Controlled Loops: Examples • Ada has a pretest version, but no posttest • FORTRAN 95 has neither • Perl and Ruby have two pretest logical loops, while and until. Perl also has two posttest loops

37. Iterative Statements: User-Located Loop Control Mechanisms break and continue • C , C++, Python, Ruby, and C# have unconditional unlabeled exits (break) • Java and Perl have unconditional labeled exits (break in Java, last in Perl) • C, C++, and Python have an unlabeled control statement, continue, that skips the remainder of the current iteration, but does not exit the loop • Java and Perl have labeled versions of continue

38. Iterative Statements: Iteration Based on Data Structures • Control mechanism is a call to an iterator function that returns the next element in some chosen order, if there is one; else loop is terminate • C's for can be used to build a user-defined iterator: for (p=root; p==NULL; traverse(p)){ }

39. Iterative Statements: Iteration Based on Data Structures (continued) • PHP • current points at one element of the array • next moves current to the next element • reset moves current to the first element • Java • For any collection that implements the Iterator interface • next moves the pointer into the collection • hasNext is a predicate • remove deletes an element • Perl has a built-in iterator for arrays and hashes, foreach

40. Iterative Statements: Iteration Based on Data Structures (continued) • Java 5.0 (uses for, although it is called foreach) • For arrays and any other class that implements Iterable interface, e.g., ArrayList for (String myElement : myList) { … } • C#’s foreach statement iterates on the elements of arrays and other collections: Strings[] = strList = {"Bob", "Carol", "Ted"}; foreach (Strings name in strList) Console.WriteLine ("Name: {0}", name); • The notation {0} indicates the position in the string to be displayed

41. Unconditional Branching • Transfers execution control to a specified place in the program • Represented one of the most heated debates in 1960’s and 1970’s • Major concern: Readability • Some languages do not support goto statement (e.g., Java) • C# offers goto statement (can be used in switch statements) • Loop exit statements are restricted and somewhat camouflaged goto’s

42. Guarded Commands • Designed by Dijkstra • Purpose: to support a new programming methodology that supported verification (correctness) during development • Basis for two linguistic mechanisms for concurrent programming (in CSP and Ada) • Basic Idea: if the order of evaluation is not important, the program should not specify one

43. Selection Guarded Command • Form if <Boolean exp> -> <statement> [] <Boolean exp> -> <statement> ... [] <Boolean exp> -> <statement> fi • Semantics: when construct is reached, • Evaluate all Boolean expressions • If more than one are true, choose one non-deterministically • If none are true, it is a runtime error

44. Loop Guarded Command • Form do <Boolean> -> <statement> [] <Boolean> -> <statement> ... [] <Boolean> -> <statement> od • Semantics: for each iteration • Evaluate all Boolean expressions • If more than one are true, choose one non-deterministically; then start loop again • If none are true, exit loop

45. Guarded Commands: Rationale • Connection between control statements and program verification is intimate • Verification is impossible with goto statements • Verification is possible with only selection and logical pretest loops • Verification is relatively simple with only guarded commands

46. Guarded Commands: Haskell • Haskell Guarded command examples f x y | y>z = ... | y==z = ... | y<z = ... where z = x*x sign x | x > 0 = 1 | x == 0 = 0 | x < 0 = -1